Ofdm reception device and ofdm reception method

ABSTRACT

An interference symbol determining section  208  compares reception power of a signal at an interference position measured by an interference position reception power measuring section  208 B with a reception power value of a desired signal measured by a desired signal measuring section  208 D for each subcarrier, thereby determines symbols which should be actually treated as interference symbols and outputs the symbols to a turbo decoding section  209 . The turbo decoding section  209  determines whether to calculate LLR values of symbols of each subcarrier signal input from a demodulation section  207  or set the LLR values to “0” based on the comparison result of the interference symbol determining section  208  and executes decoding processing.

TECHNICAL FIELD

The present invention particularly relates to an OFDM receptionapparatus and OFDM reception method used for an OFDM system according toa frequency hopping scheme.

BACKGROUND ART

In recent years, a mobile communication system based on an OFDM schemeusing frequency hopping is under study. An OFDM system using frequencyhopping carries out communication using different hopping patterns amonga plurality of cells and thereby averaging interference among cells.

A mobile communication system using a multicarrier scheme including afrequency hopping OFDM scheme adopts a coding scheme premised on MAP(Maximum a posteriori) decoding using a soft decision value such asturbo code or LDPC (Low Density Parity Check) code for coding oftransmission information.

Furthermore, turbo code decoding processing is derived on the assumptionthat a transmission path is in an AWGN (Additive White Gaussian Noise)environment and can calculate, for example, an LLR value of a turbo codefrom an LLR (Log Likelihood Ratio) calculation model shown in FIG. 1(see, for example, Wataru Matsumoto, Hideki Ochiai: “Application of OFDMModulation Scheme”, Triceps, WS No. 216 (2001-10), pp. 80).

In FIG. 1, D [received value] is set on the horizontal axis, P[probability density] is set on the vertical axis, a dotted line shows aGaussian distribution of symbol “1” when noise is received, a single-dotdashed line shows a Gaussian distribution of symbol “0” when noise isreceived and both Gaussian distributions show a variance with σ². “α” isa decision rate of symbol “0” and “−α” is a decision rate of symbol “1”indicating that P [probability density] becomes a maximum value at thesedecision rates. Furthermore, the figure shows that a probability densityP of a certain received value Drx is P1 (Drx) on the dotted line ofsymbol “1” and P0 (Drx) on the single-dot dashed line of symbol “0”.

Furthermore, distribution states of symbol “0” and “1” when noise poweris large are shown in FIG. 2. Thus, when noise power increases, thevariance width of σ2 increases and maximum values of the respectiveprobability densities P at decision rates α, −α of symbols “0” and “1”may become extremely small.

In the case of frequency hopping OFDM (hereinafter referred to as“FH-OFDM”), when collision occurs at a certain subcarrier, the symbolsof the subcarrier are believed to appear as shown in FIG. 2. In thiscase, this means that the SNR (Signal to Noise Ratio) is lower thanthose of surrounding subcarriers, and therefore errors are likely tooccur in the LLR values of symbols “0” and “1”. Furthermore, when anerror occurs in a symbol decision, the influence of this error may alsoaffect LLR values of symbols of other subcarriers.

In an environment with two cells, an example of resource assignment ofusers and pilots of a base station in the own cell is shown in FIG. 3and that in other cells is shown FIG. 4. In this regard, 1 block in thefrequency direction indicates 1 subcarrier and 1 block in the timedirection indicates 1 burst period in FIG. 3 and FIG. 4.

Normally, a user's hopping patterns and resource assignment aredetermined at random in the own cell and other cells, and thereforecollision (hit) may occur accidentally at a certain subcarrier at acertain time point. A hit situation between user 1 signals and pilotsignals in the own cell and signals in the other cell is shown in FIG.5. “0” indicates that no hit has occurred, while “1” indicates that ahit has occurred.

As described above, the turbo code and LDPC code are designed on thepremise of an AWGN channel and such a hit is not assumed, and thereforeif a hit occurs, this provokes great characteristic deterioration.

Therefore, an OFDM reception apparatus which decides thepresence/absence of error data which will provoke an error in an LLRvalue and corrects the error beforehand is proposed in, for example, theUnexamined Japanese Patent Publication No. HEI 11-355240 (hereinafterreferred to as “Patent Document 1”).

In the case of a carrier having reference data given for each carrier ora preceding symbol for differential demodulation, the amplitude of whichis smaller than a given threshold, this OFDM reception apparatus regardsthe data of the carrier as having been lost, inserts null data, outputsa soft decision sequence and carries out soft decision decoding on thesoft decision sequence using Viterbi decoding, etc. Furthermore, in thecase of a carrier greater than another given threshold, the OFDMreception apparatus regards the carrier data as having been lost,inserts null data, outputs a soft decision sequence and thereby improvesan error correcting effect when soft decision decoding is carried out.

However, the conventional OFDM reception apparatus assumes only onemodulation scheme of a PSK system, prepares two thresholds and sets anull signal (zero: 0) for a soft decision value having a high noiselevel of a received signal due to drop of fading and interference.

At this time, there may be two problems.

1) Even if an interference decision is complete, the characteristic neednot always become better depending on an amount of interference (FIG.6). This is because the decoding characteristic deteriorates when thereare too many “0”s. FIG. 6 illustrates, assuming a case where themodulation scheme is QPSK and the proportion of carriers whereinterference exists is 40%, an error rate (plot “Normal: □” in thefigure) during a calculation of a normal LLR value, error ratecharacteristic (plot “softvalue=0: ◯” in the figure) during acalculation of an LLR value when the soft decision value is assumed tobe 0 and error rate characteristic (plot “No change: X” ideal curve inthe figure) during a calculation of an LLR value when there is nointerference.

2) When adaptive modulation is used to improve data transmissionefficiency, not only PSK system but also QAM system modulation may beused. When QAM system modulation is assumed, power values of symbolsvary depending on data, and therefore it is not possible to decideinterference against two thresholds. That is, two thresholds are notenough to be applied to adaptive modulation using QAM system modulation.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an OFDM receptionapparatus and OFDM reception method capable of correctly detectinginterference positions which may actually cause deterioration of anerror rate characteristic and improving error correcting performanceduring soft decision decoding.

This object is attained by detecting a hopping position at whichinterference occurs based on hopping pattern information of, forexample, an adjacent cell or adjacent sector and a hopping pattern ofthe own cell or own sector, extracting a pilot signal from eachsubcarrier of a received frequency hopping OFDM signal, measuringreception power of a desired signal based on the extracted pilot signalsequence and known pilot signal sequence, comparing reception power ofthe signal at the detected interference position with a reception powervalue of the measured desired signal for each subcarrier, selectingwhether reception power of symbols of the respective subcarriers shouldbe used as they are or set to 0 based on the comparison result andcarrying out turbo decoding.

Thus, the reception power of the signal at the detected interferenceposition and the reception power value of the measured desired signalare compared for each subcarrier and whether the reception power ofsymbols of the respective subcarriers should be used as they are or setto 0 based on the comparison result, and therefore it is possible tocorrectly detect symbols to be treated as interference symbols andimprove error correcting performance during soft decision decoding. Thatis, it is possible to avoid symbols which are at interference positionsbut do not cause any adverse influence on an error rate characteristicwhen used for decoding from being excluded unnecessarily from decodingtargets and thereby improve error correcting performance during softdecision decoding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a conventional LLR value calculation model;

FIG. 2 illustrates a conventional LLR value calculation model under highnoise levels;

FIG. 3 illustrates an example of own cell hopping pattern;

FIG. 4 illustrates an example of other cell hopping pattern;

FIG. 5 illustrates a hit situation between user 1 signals and pilotsignals of the own cell, and signals of the other cell;

FIG. 6 illustrates an error rate characteristic when a conventional LLRvalue is calculated;

FIG. 7 is a block diagram showing the configuration of a transmissionapparatus according to Embodiment 1 of the present invention;

FIG. 8 is a block diagram showing the configuration of a receptionapparatus according to Embodiment 1;

FIG. 9 illustrates an error rate characteristic when an LLR value iscalculated (est sigma) according to Embodiment 1;

FIG. 10 illustrates an example of hopping pattern of the own cellaccording to Embodiment 2;

FIG. 11 illustrates an example of hopping pattern of the other cellaccording to Embodiment 2;

FIG. 12 illustrates a hit situation between user 1 signals of the owncell and signals of the other cell;

FIG. 13 illustrates a block diagram showing the configuration atransmission apparatus according to Embodiment 2;

FIG. 14 illustrates a block diagram showing the configuration atransmission apparatus according to Embodiment 3;

FIG. 15 schematically illustrates Expression (1) of Embodiment 3;

FIG. 16 illustrates an error rate characteristic when an LLR value iscalculated (est sigma) according to Embodiment 3;

FIG. 17 schematically illustrates Expression (2) of Embodiment 4;

FIG. 18 illustrates an error rate characteristic when an LLR value iscalculated (est sigma) according to Embodiment 4;

FIG. 19 is a flow chart illustrating the operations of an interferencesymbol determining section and turbo decoding section in a receptionapparatus according to Embodiment 5;

FIG. 20 is a flow chart illustrating the operations of an interferencesymbol determining section and turbo decoding section in a receptionapparatus according to Embodiment 6; and

FIG. 21 is a flow chart illustrating the operations of an interferencesymbol determining section and turbo decoding section in a receptionapparatus according to Embodiment 7.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the attached drawings, embodiments of the presentinvention will be explained in detail below.

Embodiment 1

With reference to the attached drawings, embodiments of the presentinvention will be explained in detail below. FIG. 7 is a block diagramshowing the configuration of a transmission apparatus according to afrequency hopping OFDM scheme (hereinafter referred to as “FH-OFDMscheme”) according to an embodiment of the present invention and FIG. 8is a block diagram showing the configuration of a reception apparatusaccording to an FH-OFDM scheme according to this embodiment related touser 1. Here, a transmission apparatus 100 is provided for a basestation and a reception apparatus 200 is provided for a communicationterminal.

The transmission apparatus 100 provided for the base station isprincipally constructed of turbo coding sections 101-1, 101-2,modulation sections 102-1, 102-2, subcarrier mapping sections 103-1,103-2, a multiplexer 104, a frequency interleave section 105, aserial/parallel (S/P) conversion section 106, an inverse fast Fouriertransform (IFFT) section 107, a guard interval (GI) insertion section108, a radio processing section 109, an antenna 110 and an adjacentinterference position notification signal generation section 111.

The turbo coding sections 101-1, 101-2 carry out turbo coding ontransmission data of user 1, user 2 and output turbo code signals to themodulation sections 102-1, 102-2.

The modulation sections 102-1, 102-2 have different code modulationfunctions and adopt a modulation scheme, for example, 16 QAM (QuadAmplitude Modulation), 64 QAM, as a QAM system or BPSK (Binary PhaseShift Keying), QPSK (Quad Phase Shift Keying), 8 PSK as a PSK system.

The modulation sections 102-1, 102-2 carry out adaptive modulationprocessing on the turbo code signals input from the turbo codingsections 101-1, 101-2 and output the modulated signals acquired to thesubcarrier mapping sections 103-1, 103-2.

The subcarrier mapping sections 103-1, 103-2 carry out mappingprocessing of mapping the modulated signals input from the modulationsections 102-1, 102-2 to their respective subcarriers according topredetermined hopping patterns and output the mapped signals to themultiplexer 104.

The outputs of the subcarrier mapping sections 103-1, 103-2, a pilotsequence and the output of the adjacent interference positionnotification signal generation section 111 are input to the multiplexer104 and the multiplexer 104 sends a serial signal resulting frommultiplexing these signals to the frequency interleave section 105.

In this embodiment, the adjacent interference position notificationsignal generation section 111 generates hopping pattern information ofthe own cell as an adjacent interference position notification signal.This allows a communication terminal to recognize a position at whichinterference occurs (that is, a subcarrier and time at whichinterference occurs) based on an adjacent interference positionnotification signal (hopping pattern information) sent from thetransmission apparatus 100 provided on a base station in an adjacentcell (other cell) and hopping pattern information of the cell (own cell)in which the communication terminal is currently located.

The frequency interleave section 105 reads the serial signal input fromthe multiplexer 104 in such a way that arrangement directions of aplurality of data sequences included in the serial signal interlace withone another and outputs the interlaced serial signal to the S/Pconversion section 106 as an interleave signal.

The IFFT section 107 applies an inverse fast Fourier transform to therespective subcarrier components of a plurality of data sequence signalsinput from the S/P conversion section 106, thereby transforms data ofthe respective subcarriers to a time domain and outputs a time waveformsignal to the GI insertion section 108.

The GI insertion section 108 inserts a guard interval for improving adelay characteristic into the time waveform signal input from the IFFTsection 107 and outputs the signal to the radio processing section 109.

The radio processing section 109 up-converts the time waveform signalinput from the GI insertion section 108 to an RF band and transmits anOFDM signal from the antenna 110.

The reception apparatus 200 shown in FIG. 8 is provided for acommunication terminal and principally constructed of an antenna 201, aradio processing section 202, a guard interval (GI) elimination section203, a fast Fourier transform (FFT) section 204, a frequencydeinterleave section 205, a channel separation section 206, ademodulation section 207, an interference symbol determining section 208and a turbo decoding section 209.

The radio processing section 202 receives the OFDM signal from theantenna 201 and outputs the OFDM signal to the GI elimination section203.

The GI elimination section 203 eliminates the guard interval from theOFDM signal input from the radio processing section 202 and outputs thesignal to the FFT section 204.

The FFT section 204 applies a fast Fourier transform (FFT) to the OFDMsignal after guard interval elimination input from the GI eliminationsection 203 and transforms the signal from the time domain to frequencydomain. This FFT section 204 extracts the data sequence signalstransmitted through a plurality of subcarriers and outputs these datasequence signals to the frequency deinterleave section 205.

The frequency deinterleave section 205 reads the plurality of datasequence signals input from the FFT section 204 in an arrangementdirection opposite to the direction in which they are interleaved in thetransmission apparatus 100, restores a serial signal including a datasequence of the original serial arrangement and outputs the serialsignal to the channel separation section 206.

The channel separation section 206 separates the serial signal includinga plurality of subcarrier signals input from the frequency deinterleavesection 205 into the respective subcarrier signals and outputs thesignal of user 1 (that is, signal directed to the own station) of thosesubcarrier signals to the demodulation section 207 and the interferencesymbol determining section 208.

The demodulation section 207 demodulates the subcarrier signals inputfrom the channel separation section 206 and outputs the demodulatedsignals to the turbo decoding section 209.

The interference symbol determining section 208 determines a symbol tobe actually treated as an interference symbol out of symbols whosehopping patterns collide with hopping patterns in an adjacent cell.

More specifically, an interference position determining section 208Adetermines a hopping position where interference occurs (that is,hopping pattern position where collision occurs between hoppingpatterns) from the hopping pattern of the adjacent cell notified and thehopping pattern of the own station and sends this interference positioninformation to an interference position reception power measuringsection 208B. The interference position reception power measuringsection 208B measures reception power at the interference position andsends the measured value to a comparison section 208E.

On the other hand, the interference symbol determining section 208extracts a pilot signal by a pilot extraction section 208C, calculatesreception power of a desired signal by a desired signal measuringsection 208D that follows based on the reception power of the pilotsignal and sends the reception power of this desired signal to thecomparison section 208E.

The comparison section 208E compares the reception power of the pilotsignal (reception power of the desired signal) with the reception powerof the symbol at the hopping position at which interference occurs foreach subcarrier, thereby determines the symbol to be actually treated asthe interference symbol and notifies the turbo decoding section 209 ofthe comparison result indicating the symbol. In the case of thisembodiment, a difference between a received signal and desired signal iscalculated for each subcarrier and when the difference is large, thesymbol is regarded as an interference symbol.

In this way, instead of simply determining an interference symbol basedon a hopping pattern, the interference symbol determining section 208instructs the turbo decoding section to carry out turbo decoding bysetting reception power of a symbol to 0 only when the symbol is locatedat an interference position based on a hopping pattern and at the sametime an SIR (Signal to Interference Ratio) is bad in consideration ofthe fact that the SIR may be good or bad due to influences of fading,etc., even at the interference position based on the hopping pattern. Asa result, even in a situation in which both interference power andreception power fluctuate due to fading, for example, in a situation inwhich both reception power and interference power values fall due tofading, interference symbols are compared and decided for eachsubcarrier, and therefore it is possible to correctly detectinterference symbols.

In this regard, power of a desired signal can be measured based on apilot signal using a method of dividing power of the received signal bypower of the desired signal for each subcarrier. Furthermore, themagnitude of the interference signal can be calculated from a differencebetween the received signal and desired signal.

The turbo decoding section 209 carries out turbo decoding by selectingwhether the reception power of symbols of each subcarrier should be usedas is or should be set to 0 based on the interference symbol position(comparison result) notified from the interference symbol determiningsection 208 and calculating an LLR value. More specifically, for symbolsof a subcarrier decided to be interference symbols by the interferencesymbol determining section 208, the turbo decoding section 209 carriesout turbo decoding by regarding the reception power thereof as 0.

FIG. 9 shows, assuming a case where the modulation scheme is QPSK andthe ratio of carriers in which interference exists is 5%, an error rate(plot “Normal: □” in the figure) when a normal LLR value is calculated,an error rate characteristic (plot “Softvalue=0: ◯” in the figure) whenan LLR value is calculated assuming that a soft decision value is 0, anerror rate characteristic (plot “No change: X” ideal curve in thefigure) when an LLR value is calculated in the case of no interferenceand an error rate characteristic (plot “est sigma: Δ” in the figure)when an LLR value is calculated by estimating an interference valueaccording to this embodiment.

Therefore, this embodiment compares the reception power of a signal at ainterference position measured by the interference position receptionpower measuring section 208B with the reception power value of a desiredsignal measured by the desired signal measuring section 208D for eachsubcarrier, decides a symbol to be actually treated as an interferencesymbol, and can thereby correctly select a symbol to be actually treatedas an interference symbol even in a fading environment, etc., andimprove the characteristic compared to the conventional method ofsetting the magnitude of σ² to the magnitude of noise when an LLR valueis calculated (Normal) or the method described in Patent Document 1(soft value=0).

Embodiment 2

Embodiment 1 above mainly assumes a PSK-based modulation scheme.According to Embodiment 1, the position of interference is set bynotifying an interference position and then the magnitude of aninterference signal is estimated from a difference between the receivedsignal and desired signal. However, it is difficult to apply this methodto a QAM-based modulation scheme in which power of each symbol of areceived signal changes depending on data.

Therefore, as shown in FIG. 10 and FIG. 11, this embodiment assumes thatthe positions of subcarriers to which a pilot sequence is assigned arethe same for the own cell and other cell and assigns sequencesorthogonal to their respective sequences. Resource assignment of usersand pilots of a base station of the own cell is shown in FIG. 10 andthat in the other cell is shown in FIG. 11.

By so doing, it is possible to measure average power of the own cell andother cell by the terminals of user 1 and user 2.

Furthermore, in this embodiment, an adjacent interference position andinterference number notification signal generation section 112 shown inFIG. 13 notifies interference positions as shown in FIG. 12 andinformation as to from which base station interference is received. Thismakes it possible to know interference positions and from which basestation interference is received with respect to a unit defined by thetime and frequency domains of user 1.

In Embodiment 1, power of interference is calculated from receptionpower of symbols in a unit where interference occurs, while in thisembodiment 2, reception power from a base station in the other cell canbe known from pilots as described above and this corresponds tointerference power, and therefore this value is reflected in thecalculation of σ² of an LLR.

Therefore, in this embodiment, calculations can be performedindependently of modulation schemes of an interference signal and adesired signal.

Embodiment 3

In Embodiments 1 and 2, an interference position can be detected bydecoding an interference position notification signal on the receivingside. This embodiment decides an interference position and estimatesinterference power by deciding power of a received signal against athreshold.

A transmission apparatus of this embodiment is shown in FIG. 14. Unlikethe transmission apparatus 100 in FIG. 7, no adjacent interferenceposition notification signal generation section 111 is connected to thetransmission apparatus 400.

Suppose the reception apparatus sets a threshold assuming that a valueobtained by subtracting power of an actually received symbol from powerof the received signal obtained from a pilot is interference signalpower+noise power per symbol and treats a symbol position exceeding thethreshold as an interference received symbol.

This interference power+noise power per symbol can be expressed by thefollowing Expression:ΔP _(r)(k,m)=(|r(k,m)|−|h(k)|·|s|)²  (1)

In Expression (1), r (k,m) denotes the mth OFDM symbol on the kthsubcarrier.

|h (k)| is magnitude of fading of a desired signal obtained from a pilotsignal and |s| is magnitude of a transmission signal. FIG. 15schematically shows Expression (1).

Assuming that the magnitude of the threshold is Tp, it is decided thatinterference has occurred when the result of Expression (1) is greaterthan the threshold Tp and the magnitude of Expression (1) is assumed tobe the value of σ² when an LLR value is calculated.

FIG. 16 shows an error rate characteristic when an LLR value iscalculated (est sigma) in this embodiment. Therefore, according to thisembodiment, it is possible to improve the characteristic compared to theconventional method (Normal) of setting the magnitude of σ² when the LLRis calculated to the magnitude of noise and the method described inPatent Document 1.

Embodiment 4

While Embodiment 3 decides interference positions using one threshold,this embodiment decides interference positions based on decision ratesof a modulation scheme and magnitude of a noise level. $\begin{matrix}{{\Delta\quad{P_{r}\left( {k,m} \right)}} = \left\{ \begin{matrix}{{{\hat{r}\left( {k,m} \right)} - {{{h(k)}} \cdot s}}}^{2} & {{{{real}\left( {\hat{r}\left( {k,m} \right)} \right)} > 0},{{{imag}\left( {\hat{r}\left( {k,m} \right)} \right)} > 0}} \\{{{\hat{r}\left( {k,m} \right)} + {{{h(k)}} \cdot s}}}^{2} & {{{{real}\left( {\hat{r}\left( {k,m} \right)} \right)} < 0},{{{imag}\left( {\hat{r}\left( {k,m} \right)} \right)} > 0}} \\{{{\hat{r}\left( {k,m} \right)} + {{{h(k)}} \cdot s^{*}}}}^{2} & {{{{real}\left( {\hat{r}\left( {k,m} \right)} \right)} < 0},{{{imag}\left( {\hat{r}\left( {k,m} \right)} \right)} < 0}} \\{{{\hat{r}\left( {k,m} \right)} - {{{h(k)}} \cdot s^{*}}}}^{2} & {{{{real}\left( {\hat{r}\left( {k,m} \right)} \right)} > 0},{{{imag}\left( {\hat{r}\left( {k,m} \right)} \right)} < 0}}\end{matrix} \right.} & (2)\end{matrix}$

FIG. 17 schematically shows Expression (2). In FIG. 17, for example,when a received signal is in a first quadrant, it is decided thatinterference exists if the received signal is outside a decision circlein the first quadrant. The magnitude of interference power+noise powerbecomes the magnitude of Expression (2) and this value is used for themagnitude of σ² when an LLR value is calculated.

FIG. 18 shows an error rate characteristic when an LLR value iscalculated (est sigma) according to this embodiment. Therefore,according to this embodiment, it is possible to improve thecharacteristic compared to the conventional method (Normal) of settingthe magnitude of σ² when the LLR value is calculated to the magnitude ofnoise and the method described in Patent Document 1.

Embodiment 5

The method of estimating interference positions and estimatinginterference power by a blind decision from reception power as describedin Embodiment 3 and Embodiment 4 above results in greater deteriorationcompared to the method of notifying interference positions andestimating interference power as in Embodiment 1 and Embodiment 2 (seethe error rate characteristics in FIG. 16, FIG. 18). Thus, thisembodiment proposes a reception apparatus capable of further improvingan error rate characteristic when the transmission apparatus 400 in FIG.14 is used (when interference positions are decided by a blind decision)compared to Embodiment 3 and Embodiment 4.

This embodiment will be explained using the reception apparatus 200 inFIG. 8 which has been explained in Embodiment 1. However, the receptionapparatus 200 of this embodiment has different processing at theinterference symbol determining section 208 because it does not receivehopping pattern information.

The operations of the interference symbol determining section 208 andturbo decoding section 209 in the reception apparatus 200 in thisembodiment will be explained using a flow chart shown in FIG. 19.

In step S101, the interference symbol determining section 208 extracts apilot signal from each subcarrier signal input from the channelseparation section 206. Next, in step S102, since the pilot sequence isknown, an inner product of the extracted pilot signal sequence and theknown pilot signal is calculated.

Next, in step S103, the interference symbol determining section 208divides the calculated inner product of the pilot sequence by the lengthof the pilot sequence to measure reception power of a desired signal.Next, in step S104, a threshold for deciding the influence of theinterference signal on the desired signal is set in consideration of thereception power value+margin of the measured desired signal.

Next, in step S105, the interference symbol determining section 208compares the reception power value with a set threshold for eachsubcarrier to decide whether the reception power value exceeds thethreshold or not. When the reception power value does not exceed thethreshold (step S105: NO), it is decided that there is no influence of avariance by interference and this fact is notified to the turbo decodingsection 209. In this case, the turbo decoding section 209 calculates anLLR value of the symbol of the subcarrier in step S106.

On the contrary, when the reception power value exceeds the threshold(step S105: YES), the interference symbol determining section 208decides that the influence of the interference on the variance is largeand notifies this to the turbo decoding section 209. In this case, instep S107, the turbo decoding section 209 sets the soft decision valueof the symbol of the subcarrier to “)”.

As shown above, the decoding processing by the interference symboldetermining section 208 and turbo decoding section 209 in the receptionapparatus 200 of this embodiment can correctly decide the influence ofinterference on symbols for each subcarrier and reliably carry out errorcorrection during soft decision decoding.

Embodiment 6

In Embodiment 5, all desired signal power is estimated using pilotsignals, but this embodiment uses a method of setting a threshold bycalculating average power of extracted symbols based on knowninterference symbols. Turbo decoding processing corresponding to thisembodiment will be explained using a flow chart in FIG. 20. Thetransmission apparatus 100 in FIG. 1 will be used as the transmissionapparatus.

In step S201, interference symbols of each subcarrier signal whichreceives interference of an adjacent cell notified by the adjacentinterference position notification signal generation section 111 out ofthe respective subcarrier signals input from the channel separationsection 206 are identified.

Next, in step S202, symbols which do not receive interference from apilot signal of the cell in which the interference symbols areidentified are extracted. Next, in step S203, average power of theextracted symbols is calculated.

Next, in step S204, a threshold for deciding the influence of theinterference signal on the desired signal is set based on the calculatedaverage power of the symbols in consideration of the averagepower+margin.

Next, in step S205, the average power value of the calculated datasignal is compared with the set threshold for each subcarrier and it isdecided whether the reception power value exceeds the threshold or not.When the reception power value does not exceed the threshold (step S205:NO), it is decided that the influence of interference on the variance issmall and LLR values of symbols of the subcarrier are calculated.

On the other hand, when the reception power value exceeds the threshold(step S205: YES), it is decided that the influence of interference onthe variance is large and the soft decision value of the symbol of thesubcarrier is set to “0” in step S207.

As shown above, according to the decoding processing by the interferencepower calculation section 208 and turbo decoding section 209 in thereception apparatus 200 of this embodiment, it is possible to correctlydecide the influence of interference on each symbol in consideration ofthe interference state for each cell and reliably carry out errorcorrection during soft decision decoding.

Embodiment 7

This embodiment will explain processing of the interference symboldetermining section 208 and turbo decoding section 209 capable ofimproving an error rate characteristic satisfactorily even when areceived signal is an adaptively modulated signal using a flow chart inFIG. 21.

Here, an environment in which PSK-based modulation (BPSK, QPSK, 8 PSK)is selected out of adaptive modulation schemes is an environment inwhich there is a great amount of interference and the SNR value issmall. In such an environment, if a threshold is set and an LLR value isset to “0” as in the case of the above described turbo decodingprocessing, the error rate characteristic deteriorates more than turbodecoding processing which reflects the amount of interference in thevalue of σ² when an LLR value is calculated without setting the softdecision value to “0”.

Furthermore, an environment in which QAM-based modulation (16 QAM, 64QAM) is selected out of adaptive modulation schemes is an environment inwhich there is less interference and Eb/N0 is large. However, in thecase of QAM modulation, there is a variation in vibration due to data,and therefore it is difficult to calculate I (interference power)+N(thermal noise power) from the received signal.

Thus, in the turbo decoding processing in FIG. 21, when the modulationscheme is a PSK system, the LLR value is calculated without setting anythreshold and when the modulation scheme is a QAM system, a threshold isset and the soft decision value is set to “0”.

In step S301 of FIG. 21, a pilot signal is extracted from eachsubcarrier signal input from the channel separation section 206. Next,in step S302, since the pilot sequence is known, an inner product of theextracted pilot signal sequence and known pilot signal is calculated.

Next in step S303, the calculated inner product of the pilot sequence isdivided by the length of the pilot sequence to measure the receptionpower of the desired signal. Next, in step S304, it is decided whetherthe modulation scheme of the received data sequence is a PSK system(BPSK, QPSK, 8 PSK, etc.) or QAM system (16 QAM, 64 QAM, etc.).

When the decided modulation scheme is a QAM system, in step S305, athreshold for deciding the influence of the interference signal on thedesired signal is set in consideration of the reception powervalue+margin of the desired signal measured in step S303.

Next, in step S306, the reception power value is compared with the setthreshold for each subcarrier and it is decided whether the receptionpower value exceeds the threshold or not. When the reception power valuedoes not exceed the threshold (step S306: NO), it is decided that thereis little influence of interference on the variance and LLR values ofsymbols of the subcarrier are calculated in step S307.

On the other hand, when the reception power value exceeds the threshold(step S306: YES), it is decided that the influence of interference onthe variance is large and the soft decision values of the symbols of thesubcarrier are set to “0” in step S308.

Furthermore, in step S304, when the decided modulation scheme is a PSKsystem, I (interference power)+N (thermal noise power) is calculated foreach symbol in step S309.

In this case, if the interference state of each cell is known, there isa received signal with no interference signal in the pilot section ineach cell, and therefore it is possible to calculate I+N for each symbolby calculating the reception power of this received signal, calculatingthe reception power corresponding to the desired signal section in thisreceived signal and subtracting the desired signal power from thereceived signal power.

Next, in step S310, the LLR value is calculated for each symbol based onI+N for each symbol calculated in step S309.

As shown above, according to the decoding processing by the interferencepower calculation section 208 and turbo decoding section 209 in thereception apparatus 200 of this embodiment, when adaptive modulationschemes such as QAM system and PSK system are used, the interferencestate is classified by modulation scheme, the influence of interferenceon symbols of each subcarrier is decided and decoding processing isperformed in such a way that the soft decision value of a symbol havinga large influence of interference is set to 0, and therefore it ispossible to correctly decide the influence of interference on eachsymbol and improve the error correcting performance during soft decisiondecoding.

In this embodiment, the transmission apparatus 400 in FIG. 14 is usedand interference positions are decided by the reception apparatus 200based on a blind decision, but it is also possible to notify theinterference positions using the transmission apparatus 100 in FIG. 7,and calculate the LLR value based on the magnitude of interferencepower.

The present invention is not limited to the above described embodimentsand can be implemented modified in various ways.

The above described embodiments have described the case where the OFDMreception apparatus and method of the present invention are applied toan FH-OFDM system with a single antenna, but the present invention isnot limited to this and effects similar to those of the embodiments canbe obtained when the present invention is applied to an FH-OFDM systemwith multiple antennas such as an MIMO (Multiple-InputMultiple-Output)-OFDM system.

Furthermore, the above described embodiments have described theapparatus and method which correctly decide symbols which should beactually treated as interference symbols even when collision of hoppingpatterns occur between mutually adjacent cells, and can thereby improvethe error correcting performance during soft decision decoding, but evenif collision of hopping patterns occurs between mutually adjacentsectors, it is possible to improve the error correcting performanceduring soft decision decoding in a similar configuration. In thisregard, the communication terminal can easily recognize collisionpositions of hopping patterns between sectors by hopping patterninformation notified from the base station of the own cell, for example.

The point is to provide an interference position determining sectionthat determines hopping positions where interference occurs based on thehopping pattern of the own sector and a hopping pattern of an adjacentsector, an extraction section that extracts a pilot signal from eachsubcarrier of the received frequency hopping OFDM signal, a measuringsection that measures reception power of a desired signal based on thepilot signal sequence extracted by the extraction section and a knownpilot signal sequence, a comparison section that compares the receptionpower of the signal at the interference position determined by theinterference position determining section with the reception power valueof the desired signal measured by the measuring section for eachsubcarrier and a decoding section that carries out turbo decoding byselecting whether the reception power of symbols of each subcarriershould be used as is or should be set to 0.

An aspect of the OFDM reception apparatus of the present inventionadopts a configuration comprising an interference position determiningsection that determines hopping positions where interference occursbased on hopping pattern information sent from a base station of anadjacent cell and the hopping pattern of the own cell, an extractionsection that extracts a pilot signal from each subcarrier of thereceived frequency hopping OFDM signal, a measuring section thatmeasures reception power of a desired signal based on the pilot signalsequence extracted by the extraction section and a known pilot signalsequence, a comparison section that compares the reception power of thesignal at the interference position determined by the interferenceposition determining section with the reception power value of thedesired signal measured by the measuring section for each subcarrier anda decoding section that decodes symbols of each subcarrier based on thecomparison result.

Another aspect of the OFDM reception apparatus of the present inventionadopts a configuration comprising an interference position determiningsection that determines hopping positions where interference occursbased on a hopping pattern of the own sector and hopping pattern of anadjacent sector, an extraction section that extracts a pilot signal fromeach subcarrier of the frequency hopping OFDM signal, a measuringsection that measures the reception power of a desired signal based on apilot signal sequence extracted by the extraction section and a knownpilot signal sequence, a comparison section that compares the receptionpower of a signal at the interference position determined by theinterference position determining section with the reception power valueof a desired signal measured by the measuring section for eachsubcarrier and a decoding section that decodes symbols of eachsubcarrier based on the comparison result.

These configurations make it possible to correctly decide influences ofinterference on each symbol of a frequency hopping scheme and improvethe error correcting performance during soft decision decoding.

In a further aspect of the OFDM reception apparatus of the presentinvention, the extraction section extracts symbols which have notreceived interference from a pilot signal of each subcarrier of thefrequency hopping OFDM signal sent from a known interference cell out ofthe plurality of cells and the comparison section calculates averagepower of the symbols extracted by the extraction section, sets athreshold for deciding the influence of the interference signal on adesired signal based on the calculated average power of the symbols andcompares the calculated average power of the data signal with the setthreshold for each subcarrier.

A still further aspect of the OFDM signal reception apparatus of thepresent invention is an OFDM reception apparatus that receives frequencyhopping OFDM signals sent from a plurality of cells, comprising anextraction section that extracts a pilot signal from each subcarrier ofthe frequency hopping OFDM signal, a measuring section that measuresreception power of a desired signal based on the pilot signal sequenceextracted by the extraction section and a known pilot signal sequence, adecision section that decides the modulation scheme of the frequencyhopping OFDM signal, a setting section that sets a threshold fordeciding the influence of the interference signal on the desired signalbased on the reception power of the desired signal measured by themeasuring section when the modulation scheme decided by the decisionsection is a QAM scheme, a comparison section that compares thereception power value with the threshold set by the setting section foreach subcarrier when the modulation scheme decided by the decisionsection is a QAM scheme, a calculation section that calculatesinterference power for each symbol based on the reception power of thedesired signal measured by the measuring section when the modulationscheme decided by the decision section is a PSK scheme and a decodingsection that carries out turbo decoding by selecting whether thereception power of the symbol of each subcarrier should be used as is orshould be set to 0 based on the comparison result when the modulationscheme decided by the decision section is a QAM scheme and carries outturbo decoding on symbols of each subcarrier based on the interferencepower calculated by the calculation section when the modulation schemedecided by the decision section is a PSK scheme.

According to this configuration, it is possible to correctly decideinfluences of interference on each symbol for each adaptive modulationscheme in consideration of the interference state and improve the errorcorrecting performance during soft decision decoding.

As explained above, the present invention can correctly detect symbolswhich should be treated as interference symbols and thereby realize anOFDM signal reception apparatus and method capable of improving theerror correcting performance during soft decision decoding.

This application is based on the Japanese Patent Application No.2003-71016 filed on Mar. 14, 2003, entire content of which is expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is suitable for use in a portable informationterminal such as a cellular phone and a base station thereof, etc.

1. An OFDM reception apparatus comprising: an interference positiondetermining section that determines hopping positions where interferenceoccurs based on hopping pattern information sent from a base station ofan adjacent cell and a hopping pattern of the own cell; an extractionsection that extracts a pilot signal from each subcarrier of thereceived frequency hopping OFDM signal; a measuring section thatmeasures reception power of a desired signal based on the pilot signalsequence extracted by said extraction section and a known pilot signalsequence; a comparison section that compares the reception power of thesignal at the interference position determined by said interferenceposition determining section with the reception power value of thedesired signal measured by said measuring section for each subcarrier;and a decoding section that carries out turbo decoding by selectingwhether the reception power of symbols of said each subcarrier should beused as is or should be set to 0 based on said comparison result.
 2. AnOFDM reception apparatus comprising: an interference positiondetermining section that determines hopping positions where interferenceoccurs based on a hopping pattern of the own sector and hopping patternof an adjacent sector; an extraction section that extracts a pilotsignal from each subcarrier of the received frequency hopping OFDMsignal; a measuring section that measures the reception power of adesired signal based on a pilot signal sequence extracted by saidextraction section and a known pilot signal sequence; a comparisonsection that compares the reception power of a signal at theinterference position determined by said interference positiondetermining section with the reception power value of a desired signalmeasured by said measuring section for each subcarrier; and a decodingsection that carries out turbo decoding by selecting whether thereception power of symbols of said each subcarrier should be used as isor should be set to 0 based on said comparison result.
 3. The OFDMreception apparatus according to claim 1, wherein said comparisonsection sets a threshold for deciding an influence of an interferencesignal on a desired signal based on the reception power of the desiredsignal measured by said measuring section and compares the receptionpower value of the desired signal measured by said measuring sectionwith said set threshold for each subcarrier.
 4. The OFDM receptionapparatus according to claim 1, wherein said extraction section extractssymbols which have not received interference from a pilot signal of eachsubcarrier of the frequency hopping OFDM signals sent from base stationsof said plurality of adjacent cells, and said comparison sectioncalculates average power of the symbols extracted by said extractionsection, sets a threshold for deciding the influence of the interferencesignal on a desired signal based on the calculated average power of thesymbols and compares the average power of said calculated data signalwith said set threshold for each subcarrier.
 5. The OFDM receptionapparatus according to claim 2, wherein said extraction section extractssymbols which have not received interference from a pilot signal of eachsubcarrier of the frequency hopping OFDM signals sent from the basestations of said adjacent sector, and said comparison section calculatesaverage power of the symbols extracted by said extraction section, setsa threshold for deciding an influence of the interference signal on thedesired signal based on the calculated average power of the symbols andcompares the average power of said calculated data signal with said setthreshold for each subcarrier.
 6. The OFDM reception apparatus accordingto claim 1, further comprising: decision section that decides amodulation scheme of a received frequency hopping OFDM signal; a settingsection that sets a threshold for deciding the influence of theinterference signal on the desired signal based on the reception powerof the desired signal measured by said measuring section when themodulation scheme decided by said decision section is a QAM scheme; acomparison section that compares the reception power value with thethreshold set by said setting section for each subcarrier when themodulation scheme decided by said decision section is a QAM scheme; anda calculation section that calculates interference power for each symbolbased on the reception power of the desired signal measured by saidmeasuring section when the modulation scheme decided by said decisionsection is a PSK scheme, wherein said decoding section carries out turbodecoding by selecting whether the reception power of the symbols of saideach subcarrier should be used as is or should be set to 0 based on saidcomparison result when the modulation scheme decided by said decisionsection is a QAM scheme and carries out turbo decoding on the symbols ofsaid each subcarrier based on the interference power calculated by saidcalculation section when the modulation scheme decided by said decisionsection is a PSK scheme.
 7. An OFDM reception method comprising: aninterference position determining step of determining hopping positionswhere interference occurs based on hopping pattern information onadjacent cells or adjacent sectors and a hopping pattern of the own cellor own sector; an extraction step of extracting a pilot signal from eachsubcarrier of the received frequency hopping OFDM signal; a measuringstep of measuring the reception power of a desired signal based on apilot signal sequence extracted in said extraction step and a knownpilot signal sequence; a comparison step of comparing reception power ofa signal at the interference position determined in said interferenceposition determining step with the reception power value of a desiredsignal measured in said measuring step for each subcarrier; and adecoding step of carrying out turbo decoding by selecting whether thereception power of the symbols of said each subcarrier should be used asis or should be set to 0 based on said comparison result.
 8. The OFDMreception apparatus according to claim 2, wherein said comparisonsection sets a threshold for deciding an influence of an interferencesignal on a desired signal based on the reception power of the desiredsignal measured by said measuring section and compares the receptionpower value of the desired signal measured by said measuring sectionwith said set threshold for each subcarrier.
 9. The OFDM receptionapparatus according to claim 2, further comprising: decision sectionthat decides a modulation scheme of a received frequency hopping OFDMsignal; a setting section that sets a threshold for deciding theinfluence of the interference signal on the desired signal based on thereception power of the desired signal measured by said measuring sectionwhen the modulation scheme decided by said decision section is a QAMscheme; a comparison section that compares the reception power valuewith the threshold set by said setting section for each subcarrier whenthe modulation scheme decided by said decision section is a QAM scheme;and a calculation section that calculates interference power for eachsymbol based on the reception power of the desired signal measured bysaid measuring section when the modulation scheme decided by saiddecision section is a PSK scheme, wherein said decoding section carriesout turbo decoding by selecting whether the reception power of thesymbols of said each subcarrier should be used as is or should be set to0 based on said comparison result when the modulation scheme decided bysaid decision section is a QAM scheme and carries out turbo decoding onthe symbols of said each subcarrier based on the interference powercalculated by said calculation section when the modulation schemedecided by said decision section is a PSK scheme.